1. Field of the Invention
This invention relates to the field of semiconductors and, more specifically, to a method and apparatus for measuring minority carrier recombination lifetimes of semiconductor materials.
2. Description of the Prior Art
Semiconductors are solid crystalline materials that are less conductive of electricity than conductors, but more conductive than insulators. Semiconductors may be comprised of single elements or compounds of multiple elements. Silicon and germanium are well known examples of elemental semiconductor materials while indium phosphide and gallium arsenide are examples of compound semiconductors.
Electrons in outer shells of atoms in conductive materials, such as metals, are not bound to specific atoms and float freely from atom to atom, so such materials conduct electric current readily, whereas electrons in outer shells of atoms in insulator materials are tightly bound to their respective atoms so that they do not conduct electric current. The ability of a semiconductor material to conduct electricity is a result of the semiconductor having negatively charged electrons and vacant electron energy states--election "holes" that behave as though they are positively charged particles near the top of an energy band. Quantum states occupied by electrons can create charge carriers with negative charges that behave as a negatively charged free particles. The positively charged holes are quantum energy states in the semiconductor material with an absence of negatively charged free particles or electrons. When an electron and a hole combine, the net charge is zero. When an electric voltage is applied across a piece of semiconductor material, a positive end of the semiconductor and a negative end of the semiconductor material are established. The externally applied electric voltage causes electrons to travel between the atoms in the semiconductor material from the negative end of the semiconductor material to the positive end of the semiconductor material, while the "holes" travel conversely in the opposite direction. The electrons and holes are referred to as charge carriers because they provide the means of charge flow, or electric current. When a voltage is applied across a piece of semiconductor material, electrons drift toward the positive end of the semiconductor and the holes drift toward the negative end of the semiconductor material.
The flow of electric current in a semiconductor can be described as motion by both electrons or holes. The semiconductor material may be "doped" by the addition of a chemical impurity to increase the number of holes or electrons. The impurities producing electrons are called donors and the impurities producing holes are called acceptors. When chemical doping is performed, the generated particle, electron or hole, is called the majority carrier. The less populous particle is called the minority carrier.
When a sample of semiconductor material is in equilibrium, no external forces such as electric voltages, electric fields, magnetic fields, or temperature gradients are acting on the semiconductor material. When in an equilibrium condition, the semiconductor material is electrically neutral with the net positive charge equal to the net negative charge. Electrons are continually being excited by heat, light, or other energy during equilibrium, however, such that electrons from lower energy bands are excited to higher energy conductor bands where they are move randomly in the semiconductor material. This "generation" of electrons for the conductor bands also generates a concomitant hole for each generated electron. Simultaneously, an electron moving randomly through the semiconductor material may come into close proximity to a hole and recombine with the hole, thus annihilating the hole. Since the net concentrations of holes and electrons in a sample of semiconductor material at equilibrium remain constant, the rate at which electron and hole pairs are generated and the rate at which they recombine must be equal.
Any deviation from equilibrium will change the electron and hole concentrations in a semiconductor to new levels. Deviation from equilibrium can be created by a net input of energy, for example, by applying an electric voltage across the semiconductor, directing light onto the semiconductor, or increasing the temperature of the semiconductor, which will increase the concentrations or densities of excess electrons and holes in the semiconductor, or by creating or allowing a net output of energy by allowing recombinations to occur without creating new electron-hole pairs at a rate equal to the recombination rate. The excess charge carriers generated by such energy input create additional electric current flowing through the semiconductor, while deletion of charge carriers by recombination inhibits electric current flow.
When sufficient energy from an external source or stimulus is applied to the semiconductor material to increase the generation of electrons and holes to a rate that is greater than the rate of recombination of electrons and holes, the population or density of electrons and holes increases until the semiconductor material reaches a new equilibrium point. The electric current carrying capacity of the semiconductor material is proportional to the densities of the electrons and holes. Therefore, increasing the densities of electrons and holes in the semiconductor material in the presence of a voltage will increase electric current flowing in of the semiconductor material.
After removal of the external energy source or stimulus from the semiconductor material the rate of recombination of electrons and holes will be greater than the rate of generation of electrons and holes until equilibrium in the semiconductor is reached. However, the semiconductor's return to its equilibrium condition will not be instantaneous. Rather, a period of time will elapse while recombination of electrons and holes occurs before the semiconductor material reaches its original equilibrium condition. During this period of time after the external energy source or stimulus to the semiconductor material has been removed and before the semiconductor material has returned to its equilibrium condition, the remaining excess charge carriers allow semiconductor to continue to conduct electricity. Thus, the longer the time period it takes a semiconductor material to return to its equilibrium condition after the externally applied energy source or stimulus is removed, the longer the semiconductor material will conduct electricity after the externally applied stimulus is removed. This important characteristic of a semiconductor is known as the semiconductor's recombination rate or minority-carrier lifetime.
Many prior art devices exist to measure a semiconductor material's minority carrier lifetime. For example, U.S. Pat. No. 5,453,703 issued to Goldfarb and U.S. Pat. No. 5,406,214 issued to Boda et al., each disclose a method or apparatus for measuring minority carrier lifetimes of semiconductor materials. Goldfarb uses a capacitance-coupling technique to measure minority-carrier recombination velocity on the surface of semiconductor materials. Unfortunately, Goldfarb's disclosed method is not suitable for testing an entire sample of semiconductor material (i.e., a bulk sample). Boda et al. measure the microwave energy reflected from holes and electrons in the specimen under test to determine minority carrier lifetimes in semiconductors. Unfortunately, the use of reflected microwaves to measure minority carrier lifetimes creates inherent limitations that limit the use of the disclosed method and apparatus, such as being limited to high resistivity samples that prevent the lifetime measurement for highly conducting materials. Other prior art devices and methods used do not provide reliable linear outputs. Therefore, such prior art devices are not accurate consistently when testing samples of different shapes, sizes, and other properties. Furthermore, such prior art devices are not always sensitive so they can not always produce measurable output signals from which the minority carrier lifetime of a sample being tested can be determined. Thus, despite the state of the art, there remains a need for a non-destructive, contact-free system for measuring minority carrier lifetimes in semiconductor materials accurately and with reliable linearity and sensitivity.